fn main() {
    println!("Hello, world!");
    // 变量的声明
    // 1、不可变的绑定变量
    let target = "world";
    // 可变的绑定对象 【mut修饰 可变】
    let mut greeting = "Hello";
    println!("{}, {}", greeting, target);
    greeting = "How are you doing";
    target = "mate";    // 不允许你再次为: target分配 值
    println!("{}, {}", greeting, target);

}
